Prehistoric population expansion in Central Asia promoted by the Altai Holocene Climatic Optimum

How climate change in the middle to late Holocene has influenced the early human migrations in Central Asian Steppe remains poorly understood. To address this issue, we reconstructed a multiproxy-based Holocene climate history from the sediments of Kanas Lake and neighboring Tiewaike Lake in the southern Altai Mountains. The results show an exceptionally warm climate during ~6.5–3.6 kyr is indicated by the silicon isotope composition of diatom silica (δ30Sidiatom) and the biogenic silica (BSi) content. During 4.7-4.3 kyr, a peak in δ30Sidiatom reflects enhanced lake thermal stratification and periodic nutrient limitation as indicated by concomitant decreasing BSi content. Our geochemical results indicate a significantly warm and wet climate in the Altai Mountain region during 6.5–3.6 kyr, corresponding to the Altai Holocene Climatic Optimum (AHCO), which is critical for promoting prehistoric human population expansion and intensified cultural exchanges across the Central Asian steppe during the Bronze Age.

weakly alkaline pH of 8-9 and a salinity of 0.08 g/L 3 , with a maximum depth of 5.6 m. The lake is surrounded by an extensive wetland, with the surrounding trees including Abies sibirica, Picea obovata, Pinus sibirica, Larix sibirica, Betula pendula, and B. pubescens.

Supplementary Note 2: Sedimentary chronology
The age model for Kanas Lake is based on accelerator mass spectrometry (AMS) 14 C dating of 7 samples of terrestrial plant remains (TPR) selected from the top 156 cm of core KNS15D (Supplementary Tab. 1). The dated materials from Kanas Lake are terrestrial plant remains (e.g., tree twigs, bark, and stem material). A previous study showed no reservoir effect for the TPR 4 , and the chronology indicates that core KNS15D spans the last ~15 kyr ( Supplementary Fig. 1a). The temporal sampling resolution for core KNS15D is 25-278 yr/cm (average ~98 yr/cm). For Tiewaike Lake, the 21 AMS radiocarbon dates are listed in Supplementary Tab. 2. Among these samples, 7 dates are from aquatic plant remains (APR) and the remaining 14 are from samples of bulk organic matter (BOM). Li et al. 3 compared the 210 Pb and AMS 14 C ages of APR from the core depth of 4.5 cm in Tiewaike Lake. The age difference was 709 yr, which was ascribed to the reservoir effect for APR; thus, we used the age of 709 yr as the reservoir effect for the APR in core TWK15A. The radiocarbon reservoir effect for the BOM was estimated by comparing the 14 C ages of APR from the depths of 250 cm, 374 cm, and 479 cm with the 14 C ages of BOM from the same depths (Supplementary Tab. 2), which indicated a highly significant linear relationship (r 2 =0.997) ( Supplementary Fig. 14). Therefore, we calculated the ages of all the BOM samples after removing different carbon reservoir effects and then established the age-depth model for Tiewaike Lake ( Supplementary Fig. 1b). The reservoir effect for the BOM samples from Tiewaike Lake shows a long-term trend of increase through the Holocene, indicating that older carbon entered Tiewaike Lake from the catchment during the middle to late Holocene, likely due to the influence of the continuously wetting climate in the southern Altai region and the resulting enhanced soil erosion 5 . The temporal sampling resolution for core TWK15A is ~4-71 yr/cm (average ~17 yr/cm).

Supplementary Note 3: Environmental interpretation of δ 30 Si diatom at Kanas Lake
During biomineralization, diatoms discriminate against the heavier 30 Si isotope in favor of the lighter 28 Si isotope, resulting in the enrichment of the residual dissolved silicon (DSi) pool in the heaver 30 Si isotope. Continued diatom growth from this increasingly isotopically-heavy dissolved silicon pool causes an increase in δ 30 Sidiatom. Therefore, changes in δ 30 Sidiatom can reflect the surface water DSi utilization in lakes 6 , assuming the lake is a closed system, which often applies to lakes with periods of strong stratification. It has been demonstrated that 28 Si is preferentially released during diatom dissolution (with a fractionation effect of -0.55 ± 0.05‰), potentially altering the measured values of δ 30 Sidiatom when the dissolution varies by >20% between individual samples 7 . No diatom dissolution is inferred for Kanas Lake due to the excellent preservation of diatoms in both the isotope samples and sedimentary diatom assemblages [8][9] , suggesting that inter-sample differences in dissolution were below the 20% threshold over the analyzed interval.
The δ 30 Sidiatom composition of sediments is often affected by multiple factors in lake systems, including changes in DSi concentrations and/or compositions caused by chemical weathering in the catchment (including clay dissolution and neo-formation of Si-minerals 10 , river/aeolian inputs, lake water residence time, changes in stratification/overturning, and other physical characteristics [11][12][13][14][15] ). These factors are usually associated with changes in climate, and it has been demonstrated that in closed lake systems (e.g., Huguangyan Marr Lake), δ 30 Sidiatom is a palaeotemperature proxy 16 . In the context of this study, we interpret the δ 30 Sidiatom signatures as indicating changes in catchment weathering and/or periods of lake overturn (intensity of stratification), and thus climate-driven processes within the Kanas Lake catchment.
The sediments from Tiewaike Lake are rich in organic matter, and some clay-rich intervals also occur. The lake water is relatively rich in humic acids derived from the peaty forest soils in the catchment. BSi was relatively low prior to 3.6 kyr (<5%) ( Supplementary   Fig. 16), when the sedimentary organic matter content was high. BSi may also be influenced by the humic acids within the lake water, in addition to climate change. The sediments of Tiewaike Lake have a low diatom concentration and there is a high proportion of Chrysophytes. This would make the potential to isolate the two forms of biogenic silica very difficult, via heavy density separation and/or sieving. Not being able to do this would effectively would adversely affect the δ 30 Sidiatom record. Our measurements of BSi indicated that the lowest values, together with the almost complete absence of diatoms, occurred before ~6.5 kyr. During ~6.5-3.6 kyr, the BSi values were moderately low and fluctuated between ~2.5 and ~4.0%. After 3.6 kyr, BSi increased (which is the opposite trend to that at Kanas Lake), suggesting that while the neoglaciation in the Altai Mountains had a major influence on Kanas Lake, it had little impact on the small, closed Tiewaike Lake basin.

Supplementary Note 4: XRF results and interpretation
We present the core-scanning X-ray fluorescence (XRF-scanning) results for cores KNS15D and TWK15A. For Kanas Lake, conventional XRF measurements were also conducted ( Supplementary Fig. 3), and for 8 elements there was a significant positive relationship (p < 0.01) between the XRF-scanning and conventional XRF results. However, the relationship for Si is negative, because conventional XRF measurements can detect biogenic silica, but XRF-scanning cannot ( Supplementary Fig. 4); therefore, SiO2 in this study was measured by conventional XRF analysis. The results of a PCA of the XRFscanning data showed that two element groups could be defined, expressed by the first two Al, Ti, Rb and K are lithogenic elements associated with the detrital mineral fraction, whereas Ca, Si and Sr represent authigenic lake production and/or allogenic inputs [17][18] .
However, Ca could be of detrital origin because it covaries with Al and Ti in the sediments of Kanas Lake. In some contexts, Si can be detrital, and BSi is derived from phytoliths, or even abiotic silica produced by the weathering of silicate minerals like quartz 19 . In Kanas Lake, SiO2 shows the opposite trend to that of the allogenic elements ( Supplementary Fig.   3), and the SiO2/Al3O2 ratio and SiO2 show similar trends (r 2 =0.94; Supplementary Fig. 6); SiO2/Al3O2 and SiO2 are strongly correlated with BSi (r 2 =0.75 and r 2 =0.65, respectively).
The silica in lakes is often of biogenic origin 19 , with BSi derived from diatoms, chrysophytes, radiolaria, and siliceous sponges. During diatom analysis, a small number of siliceous sponges were occasionally observed, but we assume that the Si record mainly reflects the productivity of the dominant diatom community.
In Kanas Lake, the Rb/Sr ratios are highly correlated with Rb (r 2 =0.68) and weakly correlated with Sr (r 2 =0.26) (Supplementary Fig. 7). The Rb/Sr ratios are therefore predominantly controlled by Rb activity during weathering within the lake watershed, indicating that the basin is mainly affected by physical erosion rather than chemical weathering [20][21] . Physical weathering increases during cold periods when the vegetation cover is reduced and greater amounts of unweathered terrestrial detrital are transported to the lake 22 . During 11.7-10.6 kyr (Geochemical Zone 2-1), Rb/Sr decreased and the sample scores on PC2 of the XRF core-scanning data increased rapidly ( Supplementary Fig. 3), indicating the increased input of coarse-grained terrigenous material, which we infer was supplied by surface runoff, most likely due to the melting of local high-altitude glaciers and permafrost with the onset of the regional climatic amelioration during the early Holocene. The trends in Sr and Zr closely track the changes in grain size ( Supplementary   Fig. 3), indicating that physical erosion in the catchment was the major source of these elements in the lake basin. This is supported by the evidence of clay-rich layers (indicative of higher concentrations of Fe and Rb) which are negatively correlated with intervals of higher Sr and Zr. Such erosional products (indicated by high concentrations of Sr and Zr) have been shown to be concentrated within the sand fraction 17 .
In the lake environment, hypolimnetic anoxia can alter the cycling of redox-sensitive elements. Although both Fe and Mn are soluble under reducing conditions, Mn is usually more soluble than Fe, and thus the Mn/Fe ratio can be used as a palaeo-redox proxy 23 24 .
Higher Mn/Fe ratios mainly reflect weaker stratification 23 , as a result of, for example, lower water level and/or higher wind speed 17,[25][26] . In Tiewaike Lake, the increased Mn/Fe ratios ( Supplementary Fig. 10) may reflect the predominantly oxidizing lake status at depth during 8.2-6.5 kyr, together with a lower water level, ignoring the possible effect of changes in wind speed. A shift to lower Mn/Fe ratios may therefore point to a lowering of the oxygen content of the bottom water during enhanced stratification due to high temperatures, and/or to deoxygenation caused by the decomposition of organic matter following the enhanced biological productivity during 6.5-3.6 kyr.

Supplementary Note 5: Sedimentary organic matter and its environmental significance
The δ 13 Corg and C/N values of modern plants obtained in previous research have revealed significant differences between aquatic and terrestrial plants 27 . High δ 13 Corg values in aquatic plants are common in lakes with longer residence times, enabling aqueous C to equilibrate with 13 C-enriched atmospheric CO2 19 . The terrestrial vegetation in the study area is dominated by C3 plants which have more negative δ 13 Corg, ranging from -37 to -24‰, with the average of -27‰ 3, 28-29 ; while for submerged plants, δ 13 Corg values vary between -20 and -12‰ 30 . Moreover, other evidence shows that the lower (more negative) the δ 13 C value, the greater the summer rainfall, and vice versa 31 . In Tiewaike Lake, δ 13 Corg indicates a mixture of aquatic and terrestrial plants. In Tiewaike Lake, an increased proportion of terrigenous organic matter would result in more negative δ 13 Corg, while an increase in the proportion of aquatic plants would result in more positive 13 Corg.
Accordingly, the co-occurrence of higher δ 13 Corg and lower C/N ratios (e.g., during ~8.2-6.5 kyr) would indicate a more significant aquatic carbon source and/or a longer lake water residence time (Fig. 2g, Supplementary Fig. 10 and Supplementary Fig. 13b). Additionally, the analysis of sediment lithology and organic matter content suggests that the abundance of aquatic plant and organic matter from its upland forest has resulted in the formation of peat sediments over time ( Supplementary Fig. 13b). The pollen data of Kanas Lake and peat investigation suggested that the forest vegetation has developed and peat has been accumulating since around 8 kyr (e.g., [4][5]32 ). Oligotrophic alpine/Arctic lakes are often Nlimited, and most of their nitrogen influx is from atmospheric sources such as precipitation and snow melt [33][34] . The δ 15 Norg values in the Tiewaike Lake sequence range between -2.2‰ and 0.94‰ (Supplementary Fig. 10), which further suggests that atmospheric nitrogen (δ 15 N ~0‰) is the dominant N source 35 . During ~8.2-6.5 kyr, the lowest δ 15 Norg values (from -0.6‰ to -2.2‰, average -1.4‰; Supplementary Fig. 10) and highest δ 13 Corg are likely to reflect a dominant aquatic source. The shallow water level and reduced detrital inputs may result in the benthos making a much larger contribution to the total lake primary production 33 . During 6.5-3.6 kyr, the high δ 15 Norg values may have resulted from increased detrital inputs (e.g., Ti, Rb, and Rb/Sr; Supplementary Fig. 10, Supplementary Fig.11 and Supplementary Fig. 12), driven by higher terrigenous productivity (the range of δ 15 N for terrestrial plants is 4.5-10.3‰ [35][36] ). Table   Supplementary  TOC corrected by subtracting the BSi content (c) for sediment core KNS15D from Kanas Lake. All records are fitted with general additive models (GAMs). The shaded bands are 95% confidence intervals. Source data are provided as a Source Data file.